If 18 YO's can't have guns does that mean that they can't trust them with guns when they go into battle as soldiers?

Eighteen year olds are considered legal adults. When the Bill of Rights was adopted, states required men aged 16 to 60 to be part of the militia. I don't know how the state will argue that preventing people under the age of 21 from purchasing long guns is worth the cost of depriving citizens of their second amendment rights.

Not sure where you're talking about, since even 14-year-olds can have guns in many US states. Can you provide more context for your question? Also, there was recent news about a Florida gun law regarding PURCHASES by 18-year-olds, but it specifically exempts anyone in the military or law enforcement, if that tells you anything. Also, the new law says nothing about possession of firearms by anyone under 21.

Difference in the Armed Forces is you get extensive training with guns before you go into battle. They don't have you show up, shove a rifle in your hands and send you to war the next day.

You get a F in paying attention kid! Buying, training, supervised use B different.

The 2nd amendment does not list any age requirements so the NRA lawsuit against Florida is ridiculous. The law says they cannot buy guns not that they cannot use them. In the military you are given pretty extensive training on gun safety, gun use, gun cleaning as well as how to shoot (I am a vet with a sharpshooter medal). How much training does the non-military gun owner of any age get?

FYI, the military trains people to use guns, that is what they do. I don't believe not be able to buy an AR-15 or similar rifle would prevent anyone from learning the basics.